Singlet Oxygen Generation and Photoinduced Charge Separation of Tetra Polyethyleneglycol Functionalized Zinc Phthalocyanine-Fullerene Dyad

Utilization of polyethyleneglycol (PEG) functionalized zinc phthalocyanine, (PEG) 4ZnPc in (i) singlet oxygen generation, and (ii) in building energy harvesting donor-acceptor systems using fullerene, C-60 as an acceptor via the well-known metal-ligand axial coordination approach is reported. The (PEG)(4)ZnPc was found to be capable of producing singlet oxygen with a quantum yield, Phi(Delta) of 0.77 in toluene, a value higher than that obtained for pristine (t-bu)(4)ZnPc (Phi(Delta) = 0.54) carrying no PEG groups, revealing its usefulness in photodynamic therapy applications. Spectroscopic studies revealed efficient binding of phenylimidazole functionalized fullerene, C(60)Im with 1:1 stoichiometry to (PEG)(4)ZnPc. Binding constant K for the formation of (PEG)(4)ZnPc: ImC(60) dyad was found to be 6x10(3) M-1 revealing moderate stability. Geometric and electronic studies of the dyad was arrived by B3LYP/3-21G(*) method. The HOMO level was found to be on zinc phthalocyanine entity while the LUMO level was found to be on the C-60 entity suggesting formation of (PEG)(4)ZnPc center dot+:ImC(60)(center dot-) charge separated state during the process of electron transfer reaction. Redox studies on the (PEG)(4)ZnPc:ImC(60) dyad enabled accurate determination of the oxidation and reduction potentials of the donor-acceptor system, and to evaluate free-energy changes associated for the charge separation process. Kinetics of photoinduced charge separation and recombination in the (PEG)(4)ZnPc:ImC(60) dyad was investigated using femtosecond transient absorption studies. Relatively long-lived charge separated states were confirmed for the dyad suggesting their potential usefulness in energy harvesting applications.